Method of hardening titanium and titanium alloys

ABSTRACT

A method of hardening the outer surface of a titanium or titanium alloy substrate under standard atmospheric conditions. The method comprises focusing an electromagnetic beam from a laser generating apparatus, absent the disposition of a chemical compound, onto at least a portion of the substrate to heat it to a point below the melting point of the substrate. The treated substrate has a substantial increased harness and durability compared to an untreated surface of titanium or titanium alloy.

FIELD OF THE INVENTION

The invention generally relates to a method for hardening the surface ofmetals or alloys; particularly, a method for hardening titanium andtitanium alloys using a laser for reducing wear and improving corrosionresistance of items made from the hardened titanium or titanium alloy.

BACKGROUND OF THE INVENTION

Titanium is an excellent lightweight material, whose strength, chemicalresistance, and biocompatibility make it particularly suitable forvarious applications in the aerospace, medical and chemical industries.However, titanium's low resistance to wear caused by sliding frictionand its low surface hardness often precludes its use in the above notedapplications. Numerous techniques have been developed to increase thesurface hardness of titanium so that it will not gall or chip off whenused in abrasive mechanical and environmental conditions (saltwater,acid, autoclave ovens, etc.)

The term “hardness” refers to the resistance of a material to localizeddeformation. Deformation includes, albeit not limited to, indentation,scratching, cutting or bending. In metals, the deformation occurs mainlyat the surface of the workpiece. There are a large variety of testingmethods used for determining the hardness of a substance, some of whichinclude the Brinell hardness test, Rockwell Hardness test, Vickershardness test and Knoop hardness test. The methodology for each of theseaforementioned tests is well known and will not be described herein forthe sake of brevity. Metallic parts exposed to abrasive and/or corrosiveconditions should be resistant to corrosion and have a surface RockwellHardness of at least 40 HRC to 55 HRC (Rockwell Hardness on the “C”scale) to prevent galling, seizing, and wear when the substrate is inrubbing and sliding contact with other materials (e.g., metal). Theapproximate surface hardness values for common materials are given belowin both Rockwell Hardness C (HRC) numbers and Knoop hardness numbers(KHN).

Material Surface Hardness 303 stainless steel 19 HRC; 180 KHNCommercially Pure titanium 16 HRC; 175 KHN Titanium alloy (Ti—6Al—4V) 34HRC; 363 KHN

Standard 303 stainless steel is prone to corrosion, whereas, thecommercially pure titanium (about 98% to about 99.5% Ti) and titaniumalloy are both resistant to corrosion. However, the low surface hardnessmakes titanium containing workpieces generally unsuitable for use inconditions where the unhardened titanium workpiece is in physicalcontact with other materials.

Surface hardening of metals is a process that includes a wide variety oftechniques designed to improve the wear resistance of parts withoutaffecting the tough interior of the part. Presently available techniquesof hardening the surface of titanium and it alloys include nitriding,anodizing, surface alloying, metallic and ceramic coatings. However,these techniques are elaborate and often require expensive equipment,such as a furnace, vacuum chamber, heat source, or other means forsupplying a specific atmosphere environment (nitrogen, argon, etc.).

Heat treating using a laser to focus onto and harden a metallicsubstrate has been utilized. Prior to using the laser, these substratesare coated or preconditioned in a manner to form a uniform layer ofoxides and/or phosphates, often referred to as “black oxidizing” to makeit more absorbent to the light of the laser. It is critical that thiscoating is uniform and substantially thick. Otherwise, a substantialportion of the laser's light energy may be reflected away from thesurface of the object. As a result of laser treating, the coatingcreates a textured layer on the workpiece with a high coefficient offriction so that the surface must be smoothed or polished, therebyadding an additional step to the process.

What has been heretofore lacking in the art is a simple and inexpensivemethod of hardening titanium and titanium alloy materials. Desirably, ahard coating is formed on the titanium which does not gall or chip offwhen in moving contact with other metal parts. It has been discovered bythe present inventor that substantial hardening of titanium and titaniumalloys is achieved with a combination of parameters on a laser. It istheorized that these specific parameters can be harmonized to create thehard surface of varying depth. The present invention could be used invarious applications, such as hardening cutting tools, hardening workingareas on hand tools, strengthening stressed areas of existing titaniumtools, etc.

DESCRIPTION OF THE PRIOR ART

Numerous patents have been directed to hardening substrates composed oftitanium and titanium alloys, however, none of the known prior artdiscloses a method of focusing a beam from a laser generating apparatusonto at least a portion of the substrate surface to harden the substratein the open, uncontrolled atmosphere.

For example, U.S. Pat. No. 5,145,530, to Cassady discloses a method ofhardening the surface of titanium and its alloys to form hard carbides,by treating the surface thereof with a moving and continuously energizedcarbon arc. A carbon arc is created by an electrical lead connected toboth an electrode, formed of carbon in any of its allotropic forms, andthe workpiece and passing an electrical current between them. The carbonarc liquefies the workpiece surface and creates craters on the workpiecesurface. The regions of the creators on workpiece are hardened.

U.S. Pat. No. 4,304,978 to Saunders is drawn to a method and apparatusutilizing a laser for heat treating a transformation hardenableworkpiece. The workpiece is initially coated with oxides or phosphatesto absorb the wavelength of the laser. Sufficiently high laser powerdensities are provided at the workpiece surface to cause an incandescentreaction with the workpiece. The incandescent reaction only occurs attemperatures above the melting point of the workpiece. In the areaswhere work-hardening has occurred a textured surface of oxide results.This must be removed by wire brushing. In the areas where work-hardeninghas not occurred hydrochloric acid must be employed to remove the oxidelayer. This in direct contrast with the present invention where thelaser heats the workpiece to point lower than the melting points of thetitanium or titanium alloy so as to avoid possible deformation of theworkpiece.

U.S. Pat. No. 4,434,189, to Zaplatynsky, is directed to coating metalsubstrates, preferably titanium and titanium alloys, by forming TiN onthe substrate surface. A laser beam strikes the surface of a movingsubstrate. Unlike the present invention, this process is performed in apurified nitrogen gas atmosphere. This heated area reacts with thenitrogen gas to form a solid solution. The alloying or formation of TiNoccurs by diffusion of nitrogen into the titanium.

U.S. Pat. No. 6,231,956 to Brenner et al., discloses a process forcreating a wear-resistant edge layer for titanium and its alloys whichcan be subjected to high loads and has a low coefficient of friction.Unlike the present invention, this process involves melting the surfaceof the substrate in a controlled atmosphere.

Therefore, there remains a need in the art for a simple andcost-effective process by which a titanium or titanium alloy workpiecemay be substantially hardened, without the need for special environmentsor pretreatments, so that the workpiece may be used in abrasivemechanical and environmental conditions.

SUMMARY OF THE INVENTION

Accordingly, the instant invention is related to a method of hardeningan outer surface of a metallic substrate under standard atmosphericconditions and without the use of inert gases or a vacuum. The methodcomprises the steps of providing a substrate of titanium or titaniumalloys and focusing an electromagnetic radiation beam formed by a lasergenerating apparatus onto at least a portion of the substrate surface toheat the substrate surface to a point below the melting point of thesubstrate and then cooling the substrate surface. The laser intensityand duration is limited such that a disposition of a chemical compoundof the surface of the substrate does not occur. The laser treatedsurface of the metallic substrate has increased hardness and durabilitycompared to the untreated surface of the substrate.

It is an objective of the instant invention to provide method oftreating a metallic substrate with a laser having a high power densityand a short exposure time such that selected areas of the substrate maybe hardened as desired under normal atmospheric conditions.

It is a further objective of the instant invention to provide a methodfor hardening a metallic substrate which does not requirepreconditioning, pretreatment, or coating prior to treatment.

Yet another objective of the instant invention to provide a method ofhardening a metallic substrate where minimum distortion and/or selectivehardening of the workpiece are achieved.

Still another objective of the invention to teach a method of hardeninga metallic substrate where the contact time of the laser on thesubstrate is sufficiently short so that no significant melting of thesubstrate occurs.

Other objects and advantages of this invention will become apparent fromthe following description taken in conjunction with any accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of this invention. Any drawings contained hereinconstitute a part of this specification and include exemplaryembodiments of the present invention and illustrate various objects andfeatures thereof.

BRIEF DESCRIPTION OF THE FIGURES

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a scanning electron micrograph (SEM) image of a cross sectionof a titanium alloy (Ti-6Al-4V) substrate magnified 200×, treated by themethod of the present invention;

FIG. 2 is a scanning electron micrograph image, at low magnification, ofthe titanium substrate of FIG. 1 mounted on a specimen stub with wire(scale bar is 1000 microns);

FIG. 3 is a detail of the surface of the titanium substrate seen in FIG.2 at a higher magnification (scale bar is 200 microns);

FIG. 4 is another SEM micrograph of the center of the titanium substrateof FIG. 1 in cross-section (scale bar is 100 microns);

FIG. 5 is an Energy Dispersive Spectrum (EDS) collected from the centralregion of the titanium substrate's cross-section illustrated in FIG. 4;

FIG. 6 is another EDS spectrum representative of fractured surfaces ofthe outer layer of the titanium substrate;

FIG. 7 is an EDS spectrum representative of surface elementalcomposition of the outer layer of the titanium substrate;

FIG. 8 is a SEM image of the area from which the spectrum presented inFIG. 6 was collected (scale bar is 5 microns);

FIG. 9 is a SEM image representative of the scan area from which theFIG. 7 surface spectrum was collected (scale bar is 20 microns)

FIG. 10 illustrates the test results for titanium alloy (Ti-6Al-4V)treated by the method of the present invention at various laserparameters, such as frequency and current;

FIG. 11 is a photograph of a bar of 303 stainless steel with a helicalgroove cut into it by a titanium cutter hardened by the presentinventive method;

FIG. 12 is a photograph of a bar of 303 stainless steel which wasattempted to be cut by a titanium cutter which was not hardened by thepresent inventive method; and

FIG. 13 is a photograph of the untreated titanium cutter used in theFIG. 12 photograph.

DETAILED DESCRIPTION OF THE INVENTION

Detailed embodiments of the instant invention are disclosed herein,however, it is to be understood that the disclosed embodiments aremerely exemplary of the invention, which may be embodied in variousforms. Therefore, specific functional and structural details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representation basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure.

It has been discovered by the present inventor that hardening oftitanium and titanium alloys is achieved using a laser generatingapparatus operating under a specific set of parameters. These parametersresult in a surface hardness that does not gall or chip off when inmoving contact with other metal parts. This makes the treated substrateparticularly desirable when used in implant or medical applications. Thehardening process is performed under open atmospheric conditions, e.g.normal air and at room temperature. Thus, no specialized equipment(vacuum chamber, gas chamber, supply of inert gas, etc.) is required.This makes the present process easier and inexpensive to perform thanthe methods taught by the prior art. Moreover, the use of a lasergenerating apparatus for hardening titanium and titanium alloys allowsfor selective treatment of the workpiece wherein only certain areas onthe surface of a workpiece can be hardened without affecting the othersurface areas thereby reducing the cost of the process.

According to a preferred, albeit non-limiting, embodiment of theinvention, surface hardening was accomplished with a Nd—YAG laser usedunder the following range of parameters. Other lasers can be used suchas a carbon dioxide laser without departing from the scope of theinvention. Utilizing a Q-switch on the laser the frequency (KHz) of thepulsed laser was from about 25 to about 50 KHz; the power or current(Amps) was from about 8 to about 25 amperes; and the speed at which thelaser beam traveled across the treated surface was from about 0.01 toabout 5.0 inches/sec (IPS). Also, the number of laser cycles, repeats ofbeam travel across the treated surface, was variable. The focal lengthof the laser was variable and dependent on the type of lens used in thelaser. Further, the use of a continuous wave (CW) or non-pulsed laserbeam also resulted in hardening of the substrate surface.

EXAMPLE 1

This is an illustrative example in which an alloy of titanium has beentreated. It is within the purview of the present invention that puretitanium or any titanium alloy could be similarly treated.

A substrate made of the Alpha-Beta alloy of titanium comprising about 6wt. % of aluminum and about 4 wt. % of vanadium (also referred to asTi-6Al-4V) with the dimensions of 10×40×60 mm3 which was cleaned by asolvent to removal all residues. The substrate was positioned on thework table under standard atmospheric conditions (temperature andpressure). A Q-switched Nd:YAG laser having a power density of60.6664×10⁴ watts/mm² at the surface of the substrate was employed. Thelaser was pulsed in the frequency range of 10-50 KHz. It was alsooperated in a continuous wave mode. The current applied was 10-20 apms.The laser beam was in constant motion so there was no “dwell time” ofthe beam on the substrate. The electromagnetic radiation beam formed bythe laser generating apparatus was focused onto a portion of the surfaceof the Ti-6Al-4V substrate. The surface was heated to a point below themelting point of the substrate. The substrate was then cooled. Thecooling could be performed by any means of cooling deemed suitable,e.g., water, air, etc. The laser-treated surface of the cooled substrateexhibited increased hardness and durability than the untreated surfaceof the substrate as evidenced by the results illustrated in the table ofFIG. 11.

In addition a sample of the titanium alloy hardened by the methoddescribed above was sent to Matco Associates, Inc. (Pittsburgh, Pa.) toperform a Knoop microhardness test to determine its surface hardness.The Knoop microhardness test was conducted at room temperature (RT). Atransverse cross-section through the coating and substrate was preparedfor subsequent metallographic inspection. In the polished condition aKnoop microhardness inspection, using a 200 gram load, was performed inthe outer 0.0015 inches of the coating as is known in the art.

Table 1 below provides the results of this Knoop microhardness test andincludes one approximate value for Rockwell Hardness C scale (HRC). Theten individual tests results shown in TABLE 1 for Rockwell Hardness Cscale (HRC) are approximate values. The resultant average Knoop hardnessnumber (KHN) of the ten tests is about 1080 and Rockwell hardness isabove about 69.7, which, as discussed above, is well above that neededto prevent galling, seizing, and wear (about 40-C to about 55-C Rockwellhardness) in the substrate when in rubbing and sliding contact withother materials.

TABLE 1 Microhardness Test KHN Approx. HRC 965 69.7 980 — 980 — 1160 —1040 — 1040 — 1190 — 1230 — 1020 — 1200 —

Analysis of Elemental Composition of Outer Coating on the TreatedTitanium of Example I.

A sample rod of the titanium alloy hardened by the aforementionedinventive procedure was submitted to Impact Analytical (Midland, Mich.)for identification of the composition of the hard surface layer. Energydispersive spectroscopy (EDS) of the central and outer areas of the rodwas performed in the scanning electron microscope (SEM), to compare theelemental composition of the surface layer (FIGS. 6 & 7) with the innerrod (FIG. 5). It was discovered that the surface layer containssignificant oxygen which is not present in the bulk rod.

The treated sample of titanium was first rinsed with acetone andmethanol, blown dry with filtered nitrogen, and tied to a SEM samplestub with wire to avoid contamination, as illustrated in FIG. 2. Theresulting specimen was inserted in the SEM at the accelerating voltageof approximately 20 keV. The EDS spectra and digital images werecollected from the outer layer and the center of the sample. Additionalspectra and images were collected from fractured surfaces of the outerlayer produced by the Knoop microhardness test. Spectra weredeconvoluted to determine elemental composition. The surface layer andbulk spectra were compared and the results are presented in TABLE 2.

FIG. 1 presents an overview of the titanium sample as mounted in thescanning electron micrograph (SEM). The photograph of the sample ismagnified at 200×, showing the coating on the titanium substrate sampleand one of the Knoop hardness indentations, after etching with Kroll'sreagent.

FIG. 2 provides further detail of the sample surface morphology. Thisfigure is a low magnification scanning electron micrograph (SEM) of thesample of titanium Ti-6Al-4V treated by the present inventive hardeningprocess. As described above, the sample is mounted on a specimen stub(not shown) with wire. Note the distinctive surface morphology of thesurface, characterized by parallel band domains with overlappingorientations.

FIG. 3 is an image detail of the surface seen in FIG. 2, at a highermagnification.

FIG. 4 is a SEM micrograph of the center of the sample bar incross-section. This is the surface area scanned for x-ray collectioncomprising the spectrum seen in FIG. 5.

FIG. 5 is the spectrum derived from an EDS collected from the centralregion of the rod cross-section shown in FIG. 4, and is representativeof the bulk rod material. SEM is used in conjunction with EDS to performelemental analysis on the microscopic section of the material being testor contaminants that may be present as is well known in the art. The EDSspectrum of FIG. 5 illustrates the x-ray energy (keV) seen along theabscissa versus the relative of counts of the detected x-rays (y-axis).The energy of the x-ray is characteristic of the element from which thex-ray was emitted. This spectrum provides both the qualitative andquantitative values for the elements present in the sample.

As seen in FIG. 5, the dominant titanium peak has been truncated, suchthat the other peaks can be scaled for visibility. Note that an overlapwith a secondary feature of Ti (K beta peak or second peak) exaggeratesthe apparent signal from vanadium. The presence of small V beta peaksupports the conclusion that vanadium is present at greater than tracelevels. The asterisk indicated a peak artifact, associated with thelarge Ti signal.

FIG. 6 is another EDS spectrum representative of fracture surfaces ofthe outer layer, providing evidence of the composition of the outerlayer without surface contamination or other variations associated withthe extreme outer surface of the coating. Comparison with the bulkspectrum in FIG. 5 reveals that oxygen is now significantly detected.This element is not present in the bulk material. The vanadium signal isagain exaggerated by overlap with Ti as noted in the FIG. 5 caption. TheTi peak artifact is again noted by an asterisk.

FIG. 7 is EDS spectrum representative of the surface elementalcomposition of the outer layer. Although the sample was cleaned withsolvents as noted above (acetone, methanol), due to the rough surfacemicrostructure some difference with the FIG. 6 spectrum may be due totrapped contamination. Aluminum (Al), Carbon (C), and Oxygen (O) aresignificantly more prevalent than in previous regions, as are severalother elements as summarized in TABLE 2 above. The vanadium signal isagain exaggerated by overlap with Ti as noted in the FIG. 5 caption. TheTi peak artifact is again noted by an asterisk.

TABLE 2 Elements detected by Energy Dispersive Spectroscopy (EDS).Sample Position Major elements Minor elements Trace elements Center(bulk) Ti Al, C, V Si Surface layer Ti Al, C, O, V Ca, Fe, Si fracturesite Surface Ti, Al, C, O Ca, Fe, Si, V Cl, K, Na, S

FIG. 8 is another SEM image of the area of the sample from which the EDSspectrum presented in FIG. 6 was collected. These regions ofmicro-fracture in the surface coating enabled the generation of thex-rays from the internal structure of the surface layer of interest.

FIG. 9 is a SEM image which is representative of the area of the samplefrom which the FIG. 7 spectrum was collected.

Observation of the sample in the stereomicroscope revealed chipped,fractured areas in the surface coating on one cross-cut end of the rodsample. These regions afforded the opportunity to gain an approximatemeasure of the layer thickness of about 60 to about 100 microns.Additionally, these fractured surfaces enables elemental analysis of theinternal composition of the outer layer. Again, significant differencesbetween the internal and surface composition of the outer layer ofinterest are noted in Table 2. Although some of the differences may bedue to contamination, trapped by the rough morphology of the externalsurface (see FIG. 2), it is unlikely that the significant increase inthe signal for aluminum, carbon and oxygen, relative to titaniumcontent, is attributable solely to contamination. The differentelemental composition at the surface of the outer layer is more likelyto originate from the layer forming process.

EXAMPLE 2

In this example, the ability of a cutting tool made from the untreatedTi-6Al-4V substrate and a cutting tool made from the same Ti-6Al-4Vsubstrate but treated by the inventive process to cut into 303 stainlesssteel were compared. FIG. 11 is a photograph of a bar of 303 stainlesssteel with a helical groove cut into it by a titanium cutter treated bythe method of the present invention. The resultant groove is about 1/16of an inch deep. FIG. 12 is a photograph of a bar of 303 stainless steelwhich was attempted to be cut by a titanium cutter which was not treatedby the method of the present invention. It can be seen that there areonly minimal abrasions on the surface of the bar. There is nopenetration into the bar as shown in FIG. 11. FIG. 13 is a photographillustrating the damage done to the untreated titanium cutter which wasused to attempt to cut the bar of 303 stainless steel shown in FIG. 12.

All patents and publications mentioned in this specification areindicative of the levels of those skilled in the art to which theinvention pertains. All patents and publications are herein incorporatedby reference to the same extent as if each individual publication wasspecifically and individually indicated to be incorporated by reference.

It is to be understood that while a certain form of the invention isillustrated, it is not to be limited to the specific form or arrangementherein described and shown. It will be apparent to those skilled in theart that various changes may be made without departing from the scope ofthe invention and the invention is not to be considered limited to whatis shown and described in the specification and any drawings/figuresincluded herein.

One skilled in the art will readily appreciate that the presentinvention is well adapted to carry out the objectives and obtain theends and advantages mentioned, as well as those inherent therein. Theembodiments, methods, procedures and techniques described herein arepresently representative of the preferred embodiments, are intended tobe exemplary and are not intended as limitations on the scope. Changestherein and other uses will occur to those skilled in the art which areencompassed within the spirit of the invention and are defined by thescope of the appended claims. Although the invention has been describedin connection with specific preferred embodiments, it should beunderstood that the invention as claimed should not be unduly limited tosuch specific embodiments. Indeed, various modifications of thedescribed modes for carrying out the invention which are obvious tothose skilled in the art are intended to be within the scope of thefollowing claims.

1. A method of hardening an outer surface of a metallic substrate inopen atmospheric conditions comprising the steps of: providing asubstrate of titanium or titanium alloys; focusing an electromagneticradiation beam formed by a laser generating apparatus onto at least aportion of said substrate surface at sufficiently high power densitiesto cause an incandescent reaction above the substrate meltingtemperature with the substrate; and limiting the incandescent reactionat any given area of the substrate to a sufficiently short period oftime to prevent any substantial melting of the substrate, whereby saidlaser treated surface of said substrate has a substantial increasedhardness and durability compared to an untreated surface of saidsubstrate.
 2. The method as set forth in claim 1, wherein said lasergenerating apparatus is operated at a frequency between about 25 toabout 50 kHz.
 3. The method as set forth in claim 1, wherein said lasergenerating apparatus operates at a power density of about 606,664watts/mm².
 4. The method as set forth in claim 1, wherein said lasergenerating apparatus operates at a scanning speed between about 0.01 toabout 5.0 inches/sec.
 5. A method of hardening an outer surface of ametallic substrate in open atmospheric conditions consisting of thesteps of: providing a substrate of titanium or titanium alloys; focusingan electromagnetic radiation beam formed by a laser generating apparatusonto at least a portion of said substrate surface at sufficiently highpower densities to cause an incandescent reaction above the substratemelting temperature with the substrate; and limiting the incandescentreaction at any given area of the substrate to a sufficiently shortperiod of time to prevent any substantial melting of the substrate,whereby said laser treated surface of said substrate has a substantialincreased hardness and durability compared to an untreated surface ofsaid substrate.
 6. The method as set forth in claim 5, wherein saidlaser generating apparatus is operated at a frequency between about 25to about 50 kHz.
 7. The method as set forth in claim 5, wherein saidlaser generating apparatus operates at a power density of about 606,664watts/mm².
 8. The method as set forth in claim 5, wherein said lasergenerating apparatus operates at a scanning speed between about 0.01 toabout 5.0 inches/sec.
 9. A titanium containing composition having ahardness in the range of 965 to 1200 Koops hardness number produced bythe process of claim
 1. 10. A titanium containing composition having ahardness in the range of 965 to 1200 Koops hardness number produced bythe process of claim 5.